Unlike charges for residential electric bills, based simply on consumption, the bills rendered for commercial and industrial installations comprise not only an actual energy consumption charge but also a demand charge. The energy usage charge is determined by metering the amount of electrical energy consumed during a billing period. The demand charge is a factor determined by the highest power consumption level during any given time interval, usually a 15 or 30 minute interval, during that billing period. Further the demand charge may have been set at some peak power usage time (usually in the summer) and be applied to the customer's bill for the next year. It is not unusual for the demand charge to represent as much as 45% of the total monthly billing. The utility companies follow this procedure because their generating installations and distribution networks must be capable of producing and supplying power at the peak demand times, irrespective of the reduced loads in the remainder of the year.
For these reasons various attempts have been made to limit the actual power consumption by industrial users and others. This is generally termed "demand limiting", although it limits the actual consumption, because it affects the demand portion of the utility bill. Initially various control manufacturers produced devices to monitor power consumption levels, and shed (that is, turn off) nonessential electrical loads as the power consumption approaches a peak demand. This brute-force approach of turn-on and turn-off is adequate for lights, small motors, and various electrical appliances that admit of such cycling control. However certain large installations, such as a centrifugal compressor motor and many large induction motors, are not designed for such on-off cycling. A centrifugal compressor, by way of example, is designed for modulating control by regulating the position of the prerotation vanes (PRV), which are sometimes termed inlet guide vanes. Regulating the vane position controls the amount of refrigerant gas admitted to the compressor, and thus the load. Regulating the large electrical motor which drives the compressor by switching it on and off to regulate the power consumption is not permissible, and in fact has led to compressor motor burnout and starting equipment failure where conventional on-off load shedding equipment has been connected in an attempt to regulate a modulating load.
It is therefore a primary object of the present invention to provide a demand regulating system where the customer installation includes a modulating electrical load, such as a centrifugal compressor. The invention will thus be described in connection with such an arrangement wherein the circuit which supplies power over the bus to the compressor motor is not changed, but a modulating signal to the motor which drives the prerotation vanes is regulated. Those skilled in the art will appreciate the application of this invention to related electrical energy systems, such as an inverter-motor combination wherein the bus voltage to the inverter is maintained but the switching frequency of the inverter thyristors is regulated to reduce the load.
When regulating a load limiting system in which a centrifugal compressor with a PRV motor to regulate the vane position is provided, it is possible to prevent further loading of the compressor by blocking the application of an increase-load signal to the PRV motor. However previous attempts to regulate load reduction have not proved feasible. For one thing, a given increment of PRV movement when the vanes are wide open does not produce a change in load equivalent to that attained by a similar movement when the vanes are already half closed. Thus a 5% change in the vane position when the vanes are initially wide open may produce virtually no effect on the load, but a 5% reduction when the vanes are already only 40% open could well send the compressor into surge and damage the system.
It is therefore another important object of the invention to provide a demand regulating system for an installation including a centrifugal compressor, in which load reduction is achieved in an effective manner over the system operating range without damaging the system.
Another important object of the invention is to provide such a system in which a plurality of slave control units, all regulated from one master unit, themselves regulate a corresponding plurality of modulating loads.
A corollary object of the invention is to provide such regulation of the plurality of modulating loads, in which the different slave units can be individually adjusted to afford load reductions at different rates in response to a single reduce signal from the master unit.
A further important object of the invention is the provision of fail-safe and alarm circuits to prevent power consumption in the event the control system loses power from the main bus, or loses the power-consumption indicating signal from the utility, or the system stays in the reduce-load mode for too long a time period, or the connection between the master unit and the slave control unit is broken.